The invention relates to a pneumatic drive system comprising at least one pneumatic drive having a drive housing and an output drive unit able to be shifted in relation to it by the action of compressed air, said output drive unit possessing an output piston, which in the drive housing divides two working chambers from each other, of which one or both is connected with control valve means serving for the controlled action of compressed air, said control valve means being able to be switched over between several switching settings which include an air economy setting defining a choke cross section.
A pneumatic drive system of this type as disclosed in the patent publication WO 02/14698 A1 is employed for crust breaking applications in aluminum processing. The system comprises a pneumatic drive designed as a crust breaker cylinder, whose output drive unit is able to be driven to perform oscillating working movements, said unit while jabbing through any crust layer, which may have accumulated, being dipped into a bath of molten aluminum for some time. A direction setting valve is responsible for the respective working movements since it controls the supply and venting of compressed air into and from two working chambers separated from each other by the output piston of the output drive unit.
Furthermore double control valve means act on the control means for the compressed air action and are placed on the connection between the direction setting valve and a respective working chamber. These control valve means may assume different switching settings, one switching setting being responsible for causing the working movement by freeing an air passage. In order to minimize use of air this switching setting is designed as an air economy setting since the fluid passage has a choke cross section which only permits a limited passage through it. Accordingly the degree of filling of the connected working chamber remains at the lowest possible level.
If the output drive unit strikes aluminum crust and is therefore subjected to a greater resistance to motion, via the choke cross section an increasing actuating pressure will be gradually built up in the connected working chamber to greater extent until the necessary penetrating force is reached. On arriving in the end of a stroke the output drive unit finally causes a switching over of the control valve means into a locked position in order to avoid further flow of compressed air into the pneumatic drive.
Owing to the time necessary for the build up of pressure in the pneumatic drive, when the output drive unit has to penetrate crust on the melt, there are irregular delays in time in the individual working cycles from case to case.
A similar arrangement is described in the European patent publication EP 0771396 B1. In this case there is also the description of an alternative design with the possibility of doing without a choke in the control lines. This however entails a continuous intense action of compressed air pressure in the working chambers, something which is a disadvantage as regards the consumption of compressed air.